outline the evidence of hypoxia playing a role in MS
Note best are Haider, EAE basic, LPS basic desai findings. and therapeutic studies.
Experimental auto-immune encephalomyelitis,EAE model hypoxia inducible factor. *Davies et al 2013)
- This involves the injection of myelin specific antibodyies into the mouse brain to induce attack and demyelination.
- These mice show disease expression ater approximately 10 days.
- They also show an EXTREMELY HYPOXIC spinal cord.
- Studies mapped the perfusion of the of the spinal cord onto heat maps and compared the changes with behvaiour.
- hypoxia correllaates temporally,spatailly and quantitatively with the extent of motor issues observed. The Cycling of the ability of the mouse to walk or have hind limb weakness correlated with reperfusion and poo perfusion. (cycled between hypoxic and enriched environments.) Hypoxia inducible factor to mark hypoxia.
LPS model inflamation link to Hypoxia and thus demyelination link to hypoxia
- These mice have have lipopolysacharide injected into their spinal cord. This induces inflamation a s major component of MS pathology.
- Here this induces a lesion that is charcateristically simmilar to the Type 3 pattern of demyelination that we observe seen early on in MS. (characteristics such as early activation of microglia also seen)
- Hence this is seen as a useful model
- The spinal cord here was extremely hypoxic ,
- Studies by Desai et al stained the LPS spinal cord and noticed increased extremely hypoxic dark circles. (ntravenous probe PIMONIDAZOLE used to mark hypoxia) These had been shown to be Oligodendrocytes that died (likely causing demyelination)
- This suggests tha inflamation may worsen hypoxia. We can support this looking at ideas of perivascular plaques, the surrounding of blood vessels by perivascular macrophages that push the brain tissue further from the nutrients and oxygen of the blood vessel.
- Studies in the LPS model injected india ink to mark the location of injection whilst adminstering LPS, anti-galC and lysolecithin.
- They found that LPS alone caused the pattern 3 like lesion no matter where it was injected. The lesion was always seen in the BASE OF THE DORSAL COLUMN.
- Evidence suggests this can be explained by an inhherent vulnerbility to hypoxia.
- Desai et al kept mice at 10% oxygen levels and then marked the spine slice for hypoxia. he found that the white matter was particuarly vulnerable. (ntravenous probe PIMONIDAZOLE used to mark hypoxia)
- taking progressive slices throughout th spine and overlapping these they tract a particuarly vulnerabilty in the base of the dorsal column
- ***– desai et al 2016 Mapping the capillaries within the dorsal spinal cord showed thaT this region had a sparse supplyy of capillaries. which mediate most oxygen perfusion. (used pERFUSION OF DIL STAINING)
- using a thoracic angiogram it was noticed that this region lied in a ARTERIAL WATER SHED. The terminal branches of the main dorsal arteriries and central ventral artery joined here. Although, beneficial in cases of single vessel occlusion in cases of whole system reduction like hypoxia the lack of anatsomotic branches means the region is severely under suplied. suggesting they would be worse effected by hypoxia.
- Identifying the vessels supplying the region show THIN LONG arteries (although most perfusion done by capilaries 40% s done by other arteries.
- In the hypoxic state these vessels lose alot of oxygen whilst they travel thei course. This was done in a study using fluroescent staining of FAD, a byrpoduct of the MRC in the Mitochondria and thus requiring oxygen, The progressive deoxygenation of this vessel can be seen. (Chisolm et al)
- Studies have found that a large proportion of surviving oligodendrocytes in lesions are perivenular, thus likley receieving oxygen from veins to sustain them.
Idea of spinal hypoxia applied to the brain
- Studies have shown the the MS brain is abnormally hypoxic reporting significant reductions in the SATURATION OF HAEMOGLOBIN)
- the circulation rate in the MS brain is also slowed going from 2.8 to 4.9 seconds. This would then further worsen the effects of hypoxia, as it gives more time for blood to become deoxygenated and will prlong the hypoxic state of cells.
- In the brain Periventrivular white matter also are supplied by long thin arteries.
- a gross study bu HAIDER et al 2016 mapped the the locations of leisions in a wide range of MS patients. He then super imposed these images finding areas of lesions in 100% of patients.
- He then mapped the location of major arterial water shed, these OVERLAPPED significantly with th location of Lesions, suggesting hypoxia is involved in damage and vulnerbaility of regions (possibly explaining SEMI-random nature of lesions.)
- A common site for lesions early on are in the Optical nerve, a site where inflamation and demyelination causes blind spots and optical neuritis. This is a again a site of a watershed.
- Wouldnt say here but could say in essay that reductions in oxygen could contribute to mitochondrial dysfunction.
Therapeutics.- these have focussed on reducing energy supply and decreasing energy requirement.
Decreasing energy requirement
- The absence of oxygen would cause in energy supply and thus reducing the energy requirement may helpo them avoid negative effects of this.
- ATP is used in the NA/K pump which is used to balance the ion greadients and pump out NA.
- methods have attempted to block NA enetry to reduce the requirement of this using NA blocckers.
- Study in EAE rats used FLECAINIDE and reported rescue of neurons.
- Clinical trial to test the use of PHENYTOIN for optic neuritis found in led to a 30% Reduction in retinal atrophy.
Increasing energy supply
- 1 method is to increased oxidative phophorylation an to do we give more oxygen.
3 studies in mice have been done here
- using a blind method to mark the neeurological scores, the placed MS models in room air and saw a decline in ability (increas in MS score)
- The placing them in 95% oxygen for an hr showed marked improvements in score,
- placing them back in room air for an hour again meadited a decline but they were still better than usually cases.
- Desai eta 2016 reports the effects of 80% oxygen envrionemnt exposure and a reduction in the volume of lesions. this was in his study of pattern 3 demyelination in LPS mice.
- a STUDY in EAE mice found that exposing mice to 75% oxygen drove improvement relative to the length of exposure with lasting benefits. peirod were 24, 48 and 72 hrs.
What are the main biological features of Multiple Sclerosis? outline the contribution of each to disease
Demyelination- l
- large feature
- responsib;e for the bulk of negative and postive symptoms.
- Primary issue caused is the loss of saltatory conduction through the removal of the myelin. This is commonly a result of oligodendrocyte death.
- The high resistance and low capacity supplied by the myelin sheat allows rapid tranduction through the internode regions to fasciliatate rapid transduction of the nerve signal.
- loss of this causes many things importantly block of propagation as the number of transporters under the myelin is not suffcient. This occuring in the Optic nerve fibre is the cause of a loss of signalling andblind spots in vision.
Inflamation-
- Another large feature
- staining fro macrophages shown the formaion of perivascular plaques. Perivscualr macrophages surround the blood vessels. The release of immune ceel it this region causies it to expansd pushing brain tissue away from the source of oxygen and nutrients contributing to hypoxia.
- Inflamation in the optic nerve is the cause of pain associate with the contralesional eye (Optical neuritis)
- Biopsy have shown the inflamation alone can cause significant clinical deficits alone (Bitsch et al 2000)
- This is thought to be a major contributor to disrupted AP propagation and mitochondrial function due to the release of induced NITRIC oxie synthase. (inflamations seen by the release of cytokines like TNF-alpha that are inudcers of iNOS)
- This is commonly ascoiated with production by microglia to produced enhanced amounts of NO to be toxic to pathogenic bacteria.
- AN ABUNDANCE of iNOS postive cells have been reported surrounding MS lesions.
- NO blocks AP conduction. This has been shown in studie stimualting neurons and recrding propagation showing a dose dependant reduction in signal with the provision of NO. This slowly fades as the NO is used up. This is linked to its action at sodium and pottasium channles.
- NO also competitively replaces Oxygen at complex IV in the MRC.hence, this could be closely related to the mitochnrial dysfunction seen in MS. this would increase the production of ROS which course mtDNA damage and oxidisa impornmat proteins and lipids and thus could be liinked to the reduction in mitcohondrial helath. Although, infalamtion contribution is not clear yet. (also NO can bind ROS to produce peroxynitrite a toxic species assscoiate with DNA damage and lipid peroxidation causing the breakdwon the Lipid membrane)
- Infalamtory mediators like TNF apha have also bee implicated in the:
- disruption of synaptic transport( along wih other changed like oedema that will alter the extracellualar fluid.)
- Glutamate is released in inflamatory lesions, amplified by TNF=alpha also by inhibitinf uptake by astrocytes.
- Youl et al 1991 implicated iflamation in optic neuritis. They showed the on the first scam 11 nerves showed increases Gadolinium, as sign of acute infalamtion. When there was recpvery there were only 2 gadolinium enhanced neurons.
- NOS has been shown to cause degeneration of axons in a much shorter exposure period than with glial species. suggests an inherent vulenrbailtiy and links inflamation to the progressive nature of the disease.
Mitochondrial dysfunction.
- The mitochondiral have a engative polarisation, hence RED positive dyes can be used to stain healthy mitochondria by electrical attraction.
- When mitochondria become dysfunctional or damged their membranes become depolarised which would prevent this.
- studies have used this to track mitochondrial health showing a significant reduction in fleuorescence in the inflamed spinal cord
- partial recovery can be seen in patients in remission.
- reductions in Mt health corrlae with reduction in ATP production which is key to AP firing and cellualr function.
Degeneration of axons.-
-Lesions cause the degenrations of axons and quite often the transection of axons resutling in a slow die back and loss of connectvity. A consequence of the other features outlined.
How can MRI be used to image damage in the MS brain?
MRI can be used to image the brain and locate lesions that appear like white blobs. These have a semi random location and can appear and dissapear in the ealy stages.
WE can see them due to the presence of Extravacualr fluid (inflamation) and thus increased intenity giving white blot.
In the progressive stages the size of lesions and numbr can be seen to grow.
outline the main symptomps of multiple sclerosis detailing the phases of MS
There are several symptoms that can be negative,LOF, or postive GOF.
Phases-
- Preclinical phase
- Within this phase there are no symptoms or attacks
- Relapse-remmtiing phase
- This can start in the mid teens and is associate with periods of symptoms and attack followed by full recovery. This is why it is usaully not diagnosed straight away as Attacks can be years apart.
- Towards the end of this phase attacks may begin to show incomplete recovery
- Secondary progressive phase
- At this stage the attacks are alot more frequent and here is little or no reocvery as the condition worsens.
- the existence of this phase as a secondary feature is a way to distinguish MS from primary progressive disease that has not relapse and remiting phase
i summary MRI activity falls with time, brain volume rtae of delcine increases, and lsion size grows.
Negative symtoms-
- contralesional Blind spots in sight (demyelination of optic nerve)
- weakness or paralysis in limbs (spinal lesions)
- loss of taste
- wobbly, loss of balance when walking
Positive symtoms
- contralesional Optic neuritis (inflamation of optic nerve)
- Uhthoffs phenomena, large LOF OF FUNCTIONING IN SIGANALLING ON HOT Days or in hot baths.
- Lhermittes phenomena- shorting pain down lower half of body when bending over. (related to demyelinated nuerons becoming mechano sensitive.
- tingling in feet
- numbness in contralesional hand.
- flashing light when eyes closed that can get bigger with moving the eye.
- ephaptic transmission, often expereince by stimuli like audition cause trigeminal pain across jaw.
Summarise Multiple sclerosis
Mutliple sclerosis in a neurodegenerative condition, consisting on lesions in the central nervous system that are charcetrised by inflamation, demyelination, mithcondrial dysfunction and degeneration of axons.
These lesions cause a wide range of loss of function and positive gain of function symptoms.
it is not inherited although predispotions can be rooted in genetics.
2 times more common in women than men.
patients suffer stages of relapse and recovery followed by a progressive late stage.
Explain in detail the cause of Uhthoffs phenomena, Lhermittes phenomena, ephaptic transmission and firzzing sensation in feet+ Paraestehsiae.
Uhthoffs phenomena, large LOF OF FUNCTIONING IN SIGANALLING ON HOT Days or in hot baths.
- This is a result of demyeliantion
- Neurons have a thing called a safety factor for firing, the amount of receptors they have a is alsways more than needed to ensure a succesufl AP can be fired. this is calculated by availabe current for AP/ the required current for AP.
- When neurons are demyelinated this leads to a lack of resistance and outflow of ions. hence the pottential gets closer to the threshold value and thus the available current is reduced. hence the safety factor falls.
- there saftey factor lies between the nonfunctional and function ranges 1.1-0.9.
- studies recording from neuons showed that heating reduced OPEN TIME of NA channels thus reducing current and this shift sit into the non functional range.
- This is supported by a stduy investigating the infleunce of cooling on the fucntion of myelinate and demeyelinated neurpns. although it has LITTLE effect on the MYELINATED neuron, It was NECCESARY for FUNCTION in the DEMYELINATED neuron.
Lhermittes-
When neurons become demyelinate then become mechanosenstive.
-the bending of the body results in the stretch and thus firing of the neurons giving the sharp sensation.
Tingling or frizzing in feet.
- recording from demyelinated neurons have shown spontaneous activit up to 50 APs a minute.
- mor eindepth study recorded from demyelinated and meylinated regions on the same neruon and again found the ectopic firing arising at the unmyelinated location. These bursts travel BIDIRECTIONALLY and those synapses in the brain are percieved as pin pricks hence tingling.
Paraesthesiae
- This has been again elated aprtly to this spontaneous activity.
- Recording from demyelinated regions of an axon showed that spike activity stimulate a burst of firing at this part of the neuron.
- hence the symptoms of a vibrating sensation when lapping hands.
Ephaptic transmission-
- This relates to the passage of a neural impulse from 1 nerve fibre to another via membrane contact.
- many times a pateint will report pain asscoiate with stimuli like audition causing pain across the lower jaw.
- This is commonly asociated with PONTINE lesions.
- This is then caused by a Auditory neruons travelling over a DEMYELINATED PONTINE NEURONS, stimualting them.
- This sensation is Laregly related to tirgeminal neurons mediating tirgeminal neuralgia.
outline research pertaining to 2 possible therapeutic strategies in multiple sclerosis?
Decreasing energy requirement
- The absence of oxygen would cause in energy supply and thus reducing the energy requirement may helpo them avoid negative effects of this.
- ATP is used in the NA/K pump which is used to balance the ion greadients and pump out NA.
- methods have attempted to block NA enetry to reduce the requirement of this using NA blocckers.
- Study in EAE rats used FLECAINIDE and reported rescue of neurons.
- Clinical trial to test the use of PHENYTOIN for optic neuritis found in led to a 30% Reduction in retinal atrophy.
Increasing energy supply
- 1 method is to increased oxidative phophorylation an to do we give more oxygen.
3 studies in mice have been done here
- using a blind method to mark the neeurological scores, the placed MS models in room air and saw a decline in ability (increas in MS score)
- The placing them in 95% oxygen for an hr showed marked improvements in score,
- placing them back in room air for an hour again meadited a decline but they were still better than usually cases.
- Desai eta 2016 reports the effects of 80% oxygen envrionemnt exposure and a reduction in the volume of lesions. this was in his study of pattern 3 demyelination in LPS mice.
- a STUDY in EAE mice found that exposing mice to 75% oxygen drove improvement relative to the length of exposure with lasting benefits. peirod were 24, 48 and 72 hrs.
outline saltatory conduction
Saltatory conduction is a method of repaid AP propagation.
- This is often refered to as the jumping of th AP along the axon.
- The high density of sodium channles at te nodes of ranvier fascilitates rapid netry and depolarisation of this region of the neruons
- The meylin proprtion f axons in the internodes then propagates this current. The high resistance and Low capactiy allowd fro rapid propagation with limit leakage of charge, each is around 1mm long.
- At the next node of ranvier the ions can finnaly ecape.
- the efflux of positve neruons rests the electrochemical gradient to induce rapid influc of ions and the process resets.
Desrcibe the mouse model for MS
EAE- experimental auto immune encephalomyelitis
- This involves injected myelin specific antiodies into the brain to drive immune attack of meylin to mimic the demyelination in MS lesions.
LPS model=
- This is a mdoel of pattern 3 spinal demyelination in MS.
- involves injections of lipopolysacahride to induce inflamation and characteristic lesion in the basal dorsal column.
What is a vascular watershed?
A vascular water shed in a location served by mutliple major arteries
This means in the case of occlusion this region is protected as it is stillserved by one artery.
However, in the case of systemic occlusion, or stemic loss of oxygenic like hypoxia, the lack of anatosomotic branches leaves this region particuarly vulnerable.
as they are served only by terminals of arteries most of the O2 supply is hevaility deoxygenated.
Why do we see this period of recovery and relapse in MS. 4 reasons
Adaptation- the brain can adapt to demteleination. Stduies have shown that demyelinated regions in lesions have increased number of axons. Although signalling is slow resulting n a functional delay this aids parital recovery.
Remyelination- Neruons can be remyelinated forming thinner and shorter inter node regions
- recording of remyelinated neruons suggests that function is largely normal.
reovery of mitchondrial health.
- studies using psotive red dye to show a decline in mitchndrial health in the MS lesioned spinal cord.
- the SAME methods have shown partial reocvery of mitochondrial health during remission (recovery of symptoms_
- Rewiring of the system
- Plastic changed can occur in neuronal wiring inorder to ry and compensate fro damge.
- This has been shown in studoies recoriding neruonal activity whilst asking MS and healthy patients to complete normal motor task reporting a much wider range of neruonal areas active in the MS patients. SUggesting they may be compensating for the LOF here.
- It has been hypothesised that feelings of fatigue in MS may be related to the spread of activity.
What underpins the eventual progressive nature of MS>
This is the result of axonal degernation. The recurrent attack and lesion causes damage and often transection to axons whih die back. As this occurs in the CNS axons cannot be regenrated and this is permanet. Over time, other recovery methids are insufficent to deal with the gowing frequency and siz eof lesions.
